1. Field of the Invention
The present invention relates to underwater acoustic hydrophone arrays. More particularly to a method for processing received underwater sonar waveforms to thereby raise the high frequency limit of the array.
2. Description of the Related Art
The gathering of data in an acoustic environment by means of an array of electrically interconnected hydrophones is important for commercial and military purposes. Passive sonar arrays have been used for many years in applications such as geologic surveys and submarine detection and tracking. They are essentially an assembly of hydrophones held together by a strength member and moved through water by a platform such as a ship or submarine to detect sound signals in the water. Such sensors are useful for detecting the position of underwater sonar target contacts. Towed arrays of hydrophones are long, continuous, neutrally buoyant hose lines, often several hundred meters long, which periodically contain numerous hydrophone receivers. Typically, these elongated arrays have a number of hydrophone elements in modules or clusters, along with their associated electronics and control leads, contained within the hose-shaped structure that extends through the water. In order to gather data in an ocean environment, one end of a linear hydrophone array is coupled to a navigating vessel. The principle advantages of the towed array are that it is separated from noise generated by the towing platform, allowing for improved detection of low source level signals, and that it can be constructed so that it is significantly longer than the towing platform, allowing for detection of lower frequency signals and for better bearing accuracy.
The individual hydrophones cooperate or interact to provide a beam, which may be steered to determine the bearing from the array of various acoustic targets. The hydrophones are connected to suitable electronics on the towing vessel which indicates the bearing of an identified underwater target. Exemplary towed arrays are described in U.S. Pat. Nos. 4,554,650 and 5,412,621. In locating submerged bodies, sonar is either used passively, through reception of signals sent through water by the target, or actively wherein a pulse of sound is sent into the water by a sonar projector. The sound is then reflected back from the target and detected by the hydrophone arrays as an echo. This echo is received as acoustic energy by an acoustic transducer, converted into electrical energy, and read by a computer on the vessel to determine the location of objects within the water. As shown in U.S. Pat. No. 5,844,860, elongated, hose-like towed arrays of hydrophones attached to the rear end of towed bodies are commonly used for the acoustic sensing of moving objects within the ocean.
In a conventional towed array, the mechanical design of the array limits the array's performance in that both the highest frequency and the lowest frequency that the array can effectively process are determined by the mechanical design itself. The lowest frequency (fL) is determined by the total array length. The fL is the frequency with half-wavelength (λ/2) equal to the length of the acoustically active portion of the array. The highest effective frequency (fH) is determined by the spacing between the individual hydrophones. The fH is the frequency with half-wavelength (λ/2) equal to the spacing between the hydrophones or hydrophone cluster.
Changes in the array performance, in either the high frequency or the low frequency, require redesigning and remanufacturing of an array which is either longer, to achieve a lower fL, or which has reduced hydrophone spacing, to raise fH. However, solving these problems by merely changing the length of the array or the hydrophone spacing may have consequences which may be undesirable. For instance, raising fH by reduction in hydrophone spacing increases the array's weight, thus complicating ballasting and buoyancy of the array and increasing the cost. Further, fH cannot be raised above the limit determined by the physical size of the hydrophone clusters being used. In addition, lowering fL requires that the acoustically active portion of the array be lengthened, increasing the complexity of the stowing and handling equipment as well as limiting the maneuverability of the towing platform.
Accordingly, the present invention solves the above mentioned problems by providing a method to process the received waveforms over a particular period of time in such a manner that the array performs as if it were synthetically longer or as if the hydrophone spacing were made synthetically shorter, thus synthetically raising the high frequency limit. The telemetry sampling frequency (fS) is chosen based on sampling requirements for the highest effective received frequency fH. Typically fS is fixed at a multiple of fH such that the desired signal waveform is adequately represented by the sample.